Code transformers fixes for latest analyzer update

pkg/code_transformers/lib/src/resolvers.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library code_transformers.src.resolvers;
 
 import 'dart:async';
 import 'package:barback/barback.dart' show AssetId, Transformer, Transform;
 
 import 'resolver.dart';
@@ skipping 50 matching lines @@
   /// Helper function to make it easy to write an `Transformer.apply` method
   /// that automatically gets and releases the resolver. This is typically used
   /// as follows:
   ///
   ///    Future apply(Transform transform) {
   ///       var entryPoints = ...; // compute entry points
   ///       return applyToEntryPoints(transform, entryPoints);
   ///    }
   Future applyToEntryPoints(Transform transform, [List<AssetId> entryPoints]) {
     return resolvers.get(transform, entryPoints).then((resolver) {
-      return new Future.value(applyResolver(transform, resolver)).then((_) {
-        resolver.release();
-      });
+      return new Future(() => applyResolver(transform, resolver))
+        .whenComplete(() {
+          resolver.release();
+        });
     });
   }
 
   /// Invoked when the resolver is ready to be processed.
   ///
   /// Return a Future to indicate when apply is completed.
   applyResolver(Transform transform, Resolver resolver);
 }